Chapter 10 - Oxygen and Hydrogen Isotopes in Rainfall-Runoff Studies

Abstract

This chapter provides an overview of traditional hydrograph separation applications on storm event time-scales, and discusses the requirements for successful separation and the results of several isotopic hydrograph separation studies. It also presents other frameworks for application of isotope data on storm-event and longer time-scales, and the findings from a number of studies in which oxygen and/or hydrogen isotopes were used to separate storm streamflow into pre-event and event water. Stable isotopes of oxygen and hydrogen have proven to be useful tools in determining the contribution of rainfall to stormflow, the residence time of water on catchments, and other aspects of watershed hydrology. The separation of contributions from event and pre-event waters to stormflow adds a constraint on streamflow generation that physical (hydrometric) measurements never could. The large contribution of pre-event water to storm streamflow is entirely consistent with the importance of subsurface stormflow as documented by hydrometric studies on forested catchments before isotopes became widely used. Pre-event water usually makes up the bulk of stormflow through some combination of mixing and displacement, even discharging to streams through macro-pores and soil pipes. The isotopes move as the water moves and do not fractionate between the time the water reaches the ground and the time the water exits the catchment as streamflow.